Example on how to cross-compile with libraries

[JD557]

Hello,

Posting my concrete issue to make things simpler, although this is a more abstract problem.

I'm trying to compile from code in my Mac OS targeting a Raspberry Pi. The thing is, my code depends on WiringPi (Scala bindings).

I don't have WiringPi on my Mac OS machine, so the final linking fails (as expected).

So it's not very clear to me how to proceed.

Maybe there's some clean way to do this with Bazel? Or maybe there's some way to stop the process just before linking, and let one perform the linking on the target machine? I don't know.

[meisam]

There are a few ways to proceed. All of them require making WiringPi available to Bazel's sandbox during build.

• Best long-term option: publish WiringPi to Bazel Central Registry
  • Benefits: WiringPi becomes available for any Bazel Project.
  • Downsides: It requires raising a pull request to https://github.com/bazelbuild/bazel-central-registry and a review by the maintainer of the bazel-central-registry project.
• Fastest approach: Build WiringPi from source
  • Benefits: It makes WiringPi available on any platform that bazel can cross-compile to, not just arm32 and arm64.
  • Downsides: It requires patching the project's source code with MODULE.bazel and BUILD.bazel files.
• Most practical: Use the existing Debian packages published with WiringPi releases on Github.
  • Benefits: Easiest setup.
  • Downsides: Unnecessary work to first build a debian package (by WiringPi maintainers), then extract it to use the libraries without installing the debian package.

[meisam]

I'm going to add an example that uses the deb package.

[meisam]

I ended up building WirinigPi from the source because it had a straightforward Makefile. The example is at

https://github.com/Programming-Rivers/rules_scala_native/tree/main/examples/04-cross-compile/04-cross-compile-dependencies

[meisam]

I verified that it works on a Pi 4B:

Build:

# ubuntu@examples/04-cross-compile/04-cross-compile-dependencies
$ bazel build //:main \
 --platforms @llvm//platforms:linux_aarch64_gnu.2.42

Copy to a Raspberry Pi:

# ubuntu@examples/04-cross-compile/04-cross-compile-dependencies
$ scp bazel-bin/main pi4:/tmp/main-gnu

Run

# pi4@/tmp
$ ./main-gnu
Initializing WiringPi...
WiringPi initialized successfully!
This program was successfully cross-compiled and dynamically linked against libwiringPi.so
Board revision (or equivalent): 2

[meisam]

I also verified that, when statically linked with musl, it can run on on a Raspberry Pi 4:
Build:

# ubuntu@examples/04-cross-compile/04-cross-compile-dependencies
$ bazel build //:main \
 --platforms @llvm//platforms:linux_aarch64_musl

Copy to a Raspberry Pi:

# ubuntu@examples/04-cross-compile/04-cross-compile-dependencies
$ scp bazel-bin/main pi4:/tmp/main-musl

Run

# pi4@/tmp
$ ./main-musl
Initializing WiringPi...
WiringPi initialized successfully!
This program was successfully cross-compiled and dynamically linked against libwiringPi.so
Board revision (or equivalent): 2

# pi4@/tmp
$ ldd main-musl
	statically linked

Compared to the dynamically linked binary to glibc:

# pi4@/tmp
$ ldd main-gnu
	linux-vdso.so.1 (0x0000007f9a16f000)
	libpthread.so.0 => /lib/aarch64-linux-gnu/libpthread.so.0 (0x0000007f99fa0000)
	libdl.so.2 => /lib/aarch64-linux-gnu/libdl.so.2 (0x0000007f99f70000)
	libm.so.6 => /lib/aarch64-linux-gnu/libm.so.6 (0x0000007f99ec0000)
	libc.so.6 => /lib/aarch64-linux-gnu/libc.so.6 (0x0000007f99d00000)
	/lib/ld-linux-aarch64.so.1 (0x0000007f9a120000)